A wireless LAN method, which was standardized as the IEEE802.11 specification, has speeded up due to the advent of IEEE802.11b, IEEE802.11 g, and the like, and the standardization of IEEE802.11n that exceeds 100 Mbps is now in progress. In IEEE802.11n, use of a MIMO (Multi-Input Multi-Output) transmission method is examined.
The MIMO transmission method is a technique that increases the transmission capacity and transmission speed in such a manner that both a sender and recipient have a plurality of antenna elements to form a plurality of logical eigen-paths by space division multiplexing (patent reference 1).
As a technique for improving the reliability of wireless transmission by obtaining a sending diversity effect using a plurality of space time block-encoded data, STBC (Space Time Block Coding) is known (patent reference 2). The STBC allows a highly reliable wireless communication, but a maximum communication speed lowers compared to a case in which no STBC is used.
Patent reference 3 describes a technique which switches between a communication using the STBC and a speed-oriented communication without using the STBC according to the priority of sending data and the state of a transmission channel. Note that patent reference 3 calculates a total of eigenvalues calculated from a channel response matrix, and determines that the state of the transmission channel is better with increasing total, and it is worse with decreasing total of eigenvalues.
Patent reference 4 describes the following technique. That is, whether or not to receive a direct wave is determined based on eigenvalues calculated from a channel response matrix. When a direct wave is received, a communication using the STBC is performed; otherwise, a speed-oriented communication without using the STBC is performed.
[Patent Reference 1] Japanese Patent Laid-Open No. 2007-019880
[Patent Reference 2] Japanese Patent Laid-Open No. 2006-074563
[Patent Reference 3] Japanese Patent Laid-Open No. 2006-333283
[Patent Reference 4] Japanese Patent Laid-Open No. 2005-039807
However, in a wireless communication system under an environment in which the influence of multi-path phasing is large, if a communication method is decided based on a temporal transmission characteristic, a communication method that is not suited to the actual environment may be selected. For example, by switching to a communication using the STBC when the transmission characteristic deteriorates instantaneously, the communication speed may lower and the throughput of the whole system may drop compared to a case in which a communication without using the STBC is continued.
The same problem may be posed when the transmission characteristic varies largely along with an elapse of time, e.g., when the sender or recipient wireless apparatus is moving rapidly.